A tissue is a mass of cells each of which performs a similar function. The four types of tissue are epithelial, connective, muscular, and nervous. An organ is a structure composed of two or more tissues which combine to perform a specific function(s). Examples of organs are the stomach, heart, lungs, skin, etc. An organ system is composed of two or more organs which combine to perform a specific function(s). Examples of organs systems are the cardiovascular system, muscular system, skeletal system, nervous system, etc.

I. Types of Tissues

There are four types of tissues: epithelial, connective, muscular, and nervous.a) Epithelial: Epithelial tissue consists of cells that cover the internal and external surfaces of the body. It functions to protect and allow for the secretion and absorbtion of substances out of and into the body.

The shapes of the cells that make-up epithelial tissue are:1. Squamous.2. Cuboidal.3. Columnar. These cells can be arranged as a single layer called simple or into multiple layers called stratified or pseudostratified. Epithelial cells may hold cilia or microvilli and can form glands that secrete their product into ducts or directly into the blood.

b) Connective: Connective tissue consists of cells dispersed in a non-cellular matrix of several types. It functions to bind other tissue types and the organs together, provides support and protection, produces blood cells, and stores fat.

c) Muscular: Muscular tissue consists of cells called muscle fibers . Muscle fibers contain filaments of the proteins actin and myosin which allow the cell to contract to produce movement.

The types of muscular tissue are:1. Skeletal or voluntary muscle – multi-nucleate, striated cells. Contraction of skeletal muscle fibers is under voluntary control. It is attached to bones via tendons.2. Smooth or involuntary muscle – single nucleus, cells are not striated. Contraction of smooth muscle fibers is involuntary. It is located in many internal organs and blood vessels.3. Cardiac muscle – single nucleus, striated, branched cells. Contraction of cardiac muscle fibers is involuntary. It is located in the heart.

d) Nervous: Nervous tissue consists of cells called neurons and neuroglia located in the brain, spinal cord, and nerves. It functions to produce and conduct nervous impulses. These nervous impulses may be sensory (internal and external stimuli), integrative (within the brain and spinal cord), and motor (to muscles and glands).